It is known in the art to modify unsaturated polyesters with rubber. However, it has been found that such compositions tend to exhibit separation when combined with a reinforcing agent or filler, thus impairing the physical properties, particularly impact strength. This separation is apparently the result of poor compatibility between the rubber and the unsaturated polyester.
Recently it has been demonstrated that this compatibility problem can be at least partially overcome by modifying the rubber to include a polar carboxyl group. However, the methods for producing the carboxy-containing rubber often result in the presence of a minor to a major proportion of alkali metal carboxylate-containing rubber as an impurity, said impurity arising from the incomplete neutralization of the alkali metal carboxylate obtained from the reaction of carbon dioxide with alkali metal-terminated polymer (living polymer). Alkali metal carboxylate-containing polymers as modifiers for unsaturated polyesters also exhibit good compatibility with the polyester and provide good, indeed outstanding physical properties. However, they suffer one major deficiency in that the viscosity of solutions of these alkali metal carboxylate-containing polymers is too high.